1. Field of the Invention
The present invention relates to a power amplifier and, more particularly, to an amplifier which uses transistors with a new structure to realize size reduction without decreasing the efficiency in a low output mode.
2. Description of the Prior Art
Generally, when a radio signal is to be transmitted from a radio device, the transmission signal is amplified by a power amplifier and output at power with a desired strength. For such a power amplifier, lower power consumption and higher efficiency are required.
As conventional amplifiers with low power consumption, there are an operational amplifier disclosed in Japanese Unexamined Patent Publication No. 1-117405, an FET amplifier circuit disclosed in Japanese Unexamined Patent Publication No. 5-243862, and an inverter amplifier disclosed in Japanese Unexamined Patent Publication No. 7-202595.
When the conventional power amplifier is used while continuously changing its output, generally, the magnitude of the bias voltage is decreased and, more specifically, in a bipolar transistor, the base voltage is lowered to decrease the output.
However, when the base voltage is lowered, the efficiency of the power amplifier lowers. The efficiency can be improved by adjusting the collector voltage.
However, to adjust the collector voltage, a larger current than that for adjusting the gate voltage is required. When power consumption for the maximum power of the power amplifier is taken into consideration, a power device with a considerably large size must be used.
Hence, the apparatus using the power amplifier excessively becomes bulky. Such a power amplifier is very disadvantageous for application to an apparatus such as a portable telephone having a limited space and requiring size reduction.
The present invention has been made in consideration of the above-described problem, and has as its object to provide a power amplifier which uses transistors with a new structure to realize size reduction without decreasing the efficiency in a low output mode.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a power amplifier for amplifying power using a transistor, wherein the transistor comprises a plurality of transistor units connected in parallel and having different static characteristics.
According to the second aspect of the present invention, there is provided a power amplifier wherein the static characteristic is a pinch-off voltage.
According to the third aspect of the present invention, there is provided a power amplifier for amplifying power using a transistor, wherein the transistor comprises a plurality of transistor units connected in parallel and having different pinch-off voltages, and when a bias voltage applied to the plurality of transistor units is lowered, predetermined transistor units of the plurality of transistor units are turned off by pinch-off, and the remaining transistor units are in an ON state.
According to the fourth aspect of the present invention, there is provided a power amplifier wherein the plurality of transistor units of the first or third aspect are connected in parallel through a matching circuit.
According to the fifth aspect of the present invention, there is provided a power amplifier wherein each of the plurality of transistor units of any one of the first, third, and fourth aspects comprises a transistor group constructed by a plurality of transistors.
According to the sixth aspect of the present invention, there is provided a power amplifier wherein the transistor group of the fifth aspect is constructed by a plurality of FETs.
According to the seventh aspect of the present invention, there is provided a power amplifier for amplifying power using a transistor, wherein the transistor comprises a plurality of transistor units connected in parallel and having different pinch-off voltages, each of the plurality of transistor units comprises a transistor group having a plurality of transistors, when a bias voltage applied to the plurality of transistor units is lowered, predetermined transistor units of the plurality of transistor units are turned off by pinch-off, and the remaining transistor units are in an ON state, a size of the transistor group of each transistor unit turned off by pinch-off when the bias voltage is lowered is set to cover output power in a high power output mode, and a size of the transistor group of each transistor unit in the ON state when the bias voltage is lowered is set to cover output power in a low power output mode, and the amplifier is designed to increase an efficiency of the transistor group in the ON state when the bias voltage is lowered in the low power output mode.
According to the eighth aspect of the present invention, there is provided a radio device using the power amplifier of any one of the first, third, and fourth aspects as an amplifier for amplifying a radio transmission output.
According to the ninth aspect of the present invention, there is provided a portable telephone using the power amplifier of any one of the first, third, and fourth aspects as an amplifier for amplifying a radio transmission output.
According to the 10th aspect of the present invention, there is provided a portable telephone comprising an antenna for transmitting/receiving a radio signal to/from a base station, a transmission/reception demultiplexer for demultiplexing transmission/reception for the antenna, a first amplifier for amplifying a reception signal from the transmission/reception demultiplexer, a first filter for filtering an output from the first amplifier, a first synthesizer for outputting a signal having a predetermined frequency, a first mixer for synthesizing an output from the first filter with an output from the first synthesizer, a second filter for filtering an output from the first mixer, a second amplifier for amplifying an output from the second filter, a second synthesizer for outputting a signal having a predetermined frequency, a demodulator for demodulating an output from the second amplifier using an output from the second synthesizer, a baseband processing section for inputting the signal demodulated by the demodulator and performing control associated with transmission/reception, a loudspeaker for generating a received voice signal controlled by the baseband processing section, a microphone controlled by the baseband processing section to collect a voice of a user, a third synthesizer for outputting a signal having a predetermined frequency, a modulator for modulating a signal from the baseband processing section using an output from the third synthesizer, a third amplifier for amplifying an output from the modulator, a third filter for filtering an output from the third amplifier, a second mixer for synthesizing an output from the third filter with an output from the first synthesizer, a fourth filter for filtering an output from the second mixer, a fourth amplifier for amplifying an output from the fourth filter and outputting the output to the transmission/reception demultiplexer, a first D/A converter for converting a first amplification factor adjustment signal from the baseband processing section into an analog signal and supplying the amplification factor adjustment signal to the second amplifier to adjust an amplification factor of the second amplifier, and a second D/A converter for converting a second amplification factor adjustment signal from the baseband processing section into an analog signal and supplying the amplification factor adjustment signal to the fourth amplifier to adjust an amplification factor of the fourth amplifier, wherein the power amplifier of one of the first, third, and fourth aspects is used as the fourth amplifier.
The power amplifier of the present invention is realized by connecting a plurality of transistor units in parallel on one semiconductor chip. When all of the bases, emitters, and collectors of the transistor units (gates, sources, and drains of FETs) are connected, and a DC operating point is changed, the operating points of all the transistor units also change.
In the present invention, each of the plurality of transistor units connected in parallel is constructed by a group of some transistors. The groups have different static characteristics and, preferably, different pinch-off voltages of FETs. With this arrangement, when the bias voltage is lowered, certain groups are set in a pinch-off state to completely reduce power consumption to zero, and the remaining groups are set in an ON state. When the size of transistors of the groups which are turned off by pinch-off first is increased, output power in the high power output mode can be maintained. When the amplifier is designed to increase, in the low output mode, the efficiency of the transistors of the groups which are not turned off by pinch-off in the low power output mode, the efficiency in the low output mode can be improved.
In the present invention, the object of the present invention can also be realized by forming transistors having the conventional size in the conventional number and adding a plurality of transistors having a small size and different static characteristics.
According to the present invention, when the power amplifier is used while continuously changing its output, complex control need not be performed, the output power has no discontinuous points, and the efficiency in the low output mode can be improved.
In addition, according to the present invention, since a circuit for adjusting the drain voltage in accordance with the change in bias voltage can be omitted, the circuit can be simplified and made compact, and it can be used in a device such as a portable telephone requiring size reduction.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.